The heat is intolerable in the summer, and the city has no defining cultural tradition or obvious reason for existence. Still, people keep coming: Phoenix is now the nation's fifth-largest city, most recently surpassing Philadelphia.

Ninety percent of the city has been built since 1950. Developers sold an affordable outdoor lifestyle to folks moving to the Arizona desert, especially from the cold upper-Midwest.

The marketing worked: The Phoenix metro area -- almost 4 million people -- is made up of about two dozen separate cities. Phoenix itself now boasts the same population as San Diego and an area larger than Los Angeles. Neighboring Mesa has the population of Pittsburgh; Tempe is as big as Kansas City.

Phoenix is a place where families, immigrants and retirees go to reinvent themselves, unencumbered by history and tradition. But they bring with them some of the same problems -- rising costs and traffic jams -- that they came to Phoenix to escape.

In the first of a three-part series, we look at how Phoenix created the ideal middle-class lifestyle -- and what could ruin it.